I have lived in San Diego all of my 67 years and the riding is as good as anywhere. But you have go to the backcountry if you want scenic challenging rides. Palomar is raw climbing and can be a bit boring.
Here is a suggestion that has some of best roads in the country.
Some are steep and remote if that is what you like.
You could easily get lost and there may be no cell coverage.
https://ridewithgps.com/routes/45081867
This is the ideal time of year to do this as it gets too hot in the summer.
Like others have said you can do the coast to Oceanside.
Very popular route lots of traffic lights but you are next to the ocean.
I live on the coast and find it somewhat boring.
Have fun.